This invention relates to a circularly polarized broadcast television antenna having crossed dipoles arrayed about a support mast.
Television transmission standards have long required horizontally polarized broadcast transmission. In horizontal polarization, the electric (E) vector of the transmitted TEM wave is oriented horizontally. It has been proposed that television reception might be improved for the average viewer if the broadcast signal were circularly-polarized (CP) rather than horizontally polarized. In CP, two orthogonal planes of polarization are excited at the same frequency but with a 90.degree. or quarter-wavelength (.lambda./4) displacement between the polarizations. This results in an electric vector which in effect rotates at the carrier frequency as it propagates. Some of the advantages of CP reception to the viewer are stated to be ease in adjusting rabbit-ear antennas and, under some circumstances, a reduction in ghosting resulting from multipath transmission.
Broadcast antennas for generating circular polarization are known. For example, U.S. Pat. No. 4,011,567 issued Mar. 8, 1977 to Ben-Dov describes a broadcast antenna for producing CP radiation. This antenna uses a circular array of helices wound about and driven relative to a support mast.
It is also known to use slanted dipoles (dipoles oriented at an angle of 45.degree. from the vertical) in a circular array about a central support mast for generating CP. Each dipole thus oriented produces an E-vector at a 45.degree. to the support mast. This E-vector may be resolved into vertical and horizontal components which propagate away from the dipole. The horizontally polarized component is virtually unaffected by the presence of the support mast, but the vertically polarized component interacts with the mast. This interaction leads to reradiation by the mast, possibly along its entire length. The field reradiated by the mast adds vectorially to the vertical component of the field radiated directly by the slanted dipole. Since the mast has a large aperture, the reradiated field varies sharply in magnitude with observation angle, and therefore the sum field will exhibit irregular peaks and nulls which adversely affect the perfection of the circular polarization (also known as axial ratio or AR).
Other arrangements for generating circular polarization are known. For example U.S. Pat. No. 4,109,255 to Silliman describes pairs of bent dipoles or helical loops fed in phase opposition to produce omnidirectional radiation which is circularly polarized. In normal use, such antennas are mounted alongside a support tower, and the degradation of the vertically polarized portion of the radiation pattern is accepted.
A simple and inexpensive transmitting antenna is desired which is circularly polarized in the presence of its support structure and which has low wind loading.